Darwin’s primary goal in The Descent of Man was to convince his readers that the general principles of the evolutionary theory he had laid out in The Origin of Species were equally applicable to humankind as they were to “lower” forms of life – namely, that our species, like all others, was descended from another, preexisting species, and that this process had been accomplished in large part (though not solely) through the agency of natural selection. Conspicuously absent from Descent was any account of the actual forms through which humankind’s line of evolutionary descent had passed. The reason for this was simply that, on the matter of human evolution, the fossil record remained silent, and Darwin was too cautious a scientist to venture into lines of argument for which he saw little supporting evidence. He also knew that the sparseness of the fossil record was not in itself sufficient reason to reject evolution. So, in the absence of any fossilized remains of ancestors, Darwin restricted himself to such genealogical inferences as could be made by comparing humans to other living forms in the light of his evolutionary principles.

As convincing as Darwin may have been using the evidence he had at hand, absence breeds curiosity. The scientific study of human evolution after Darwin has been animated in large part by the desire for the direct, material evidence of our species’ evolutionary ancestry that was still lacking at the time of Darwin’s death in 1882. Pervasive talk of “missing links” throughout the twentieth century testified to the hold that the absent ancestors had on both professional and public minds: Darwin had shown in principle that humans had an evolutionary history, but now the task was to populate that history. This is not to say that the work done by students of human evolution since Darwin has been only to slot newly discovered fossil ancestors into a theoretical framework set in stone by the Great Man. Quite the contrary, Darwin’s model of human evolution was challenged, defended, and modified on a number of fronts simultaneously to the influx of previously unknown fossil evidence.